The invention relates to an apparatus for converting a direct current into an alternating current, comprising a direct-current converter having an input and an output, which direct-current converter is continuously controllable, a capacitor, a control unit for controlling the direct-current converter, and an unfolding bridge.
An apparatus of this kind is known in various embodiments thereof. When direct-current is being converted into alternating current, the current and the voltage on the direct-current side of the converter must be practically constant, and consequently also the power is practically constant. On the alternating current side of the converter the current and the voltage are sinusoidal, so that the momentaneous power depends on the time in that case and assumes values between zero and twice the average power. This is only possible if it is possible to store energy in the direct-current converter.
A prior art solution for the storage of energy in the direct-current converter of the type referred to above is to connect a capacitor to the direct-current side of the converter. The unfolding bridge is connected to the alternating current side and ensures that the output voltage of the direct-current converter will be positive at all times. The output voltage of the direct-current converter thereby equals the absolute value of the ac voltage on the output of the unfolding bridge. With this prior art solution, the switching frequency of the switches in the unfolding bridge is twice the frequency of the ac output voltage, which is usually 50 Hz or 60 Hz, so that the switching losses of the unfolding bridge are negligible, due to the relatively low frequency. The drawback of this prior art solution is that it can only be used in a satisfactory manner if the input direct-current voltage is relatively high. The ripple current and the ripple voltage decrease proportionally to the magnitude of the capacitor of the capacitor. The energy stored in a capacitor is E=1/2CU.sup.2. This means that when the input voltage is reduced by a factor k, the capacity of the capacitor must increase by a factor k.sup.2 in order to eliminate the ripple to the same degree. This leads to larger dimensions and a higher cost of the capacitor.
An alternative prior art solution for the energy storage is to connect the capacitor to the alternating current side of the direct-current voltage converter, so that the energy storage will take place at a high voltage at all times and the capacitor can remain small. The conversion of direct-current into alternating current will then take place in a second stage. One drawback of this prior art solution is the fact that conversion takes place twice, causing a lower efficiency of the conversion.
The object of the invention is to provide apparatus of the kind referred to in the introduction, wherein the efficiency of the conversion will be high and wherein a relatively small capacitor will suffice.